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Comment by microtonal | Hacker News Reader

NeXTSTEP forked Mach at version 2.5

Various sources state that they rebased to OSF Mach Kernel 7.3, which was based on Mach 3 and parts of Mach 4. The OSF MK ancestry of macOS XNU can still be seen in paths:

https://github.com/apple-oss-distributions/xnu/tree/main/osf...

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This is quite a ship-of-Theseus problem. Steve Jobs hired Avi Tevanian to work at NeXT on the Mach kernel, and later brought Tevanian to Apple; he was still working on the Mach/Darwin kernel as late as the first iPhone release. There is a reasonable argument to be made that the OSF kernel is actually the derivative, by default.

There are a few references to the OSF Mach kernel from '98 in a few files, such as https://github.com/apple-oss-distributions/xnu/blob/main/osf...

EDIT: Apple calls out Mach 3.0.

https://developer.apple.com/library/archive/documentation/Da...

Apple took Mach 3.0 and collapsed it into a single address space for performance purposes. Certainly reasonable.

As far as I can tell (this isn't gospel, just something I've inferred as the only reasonable explanation from comparing the codebases), Apple reconstructed a Mach 2.5 style kernel, like NeXTSTEP had, from fresh codebases (OSF Mach and 4.4BSD-Lite2), perhaps because 4.3BSD was still encumbered at the time.

The Darwin 0.1 and 0.3 releases contain the old kernel, derived directly from NeXTSTEP, and that's the direct derivative of Mach 2.5. The later XNU appears to be a reconstruction of that kernel with unencumbered code and that's also when IOKit replaced DriverKit.

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